Referring to FIG. 1, a gas turbine engine is generally indicated at 10 and comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high pressure compressor 14, combustion equipment 15, a high pressure turbine 16, an intermediate pressure turbine 17, a low pressure turbine 18 and an exhaust nozzle 19.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 which produce two air flows: a first air flow into the intermediate pressure compressor 13 and a second air flow which provides propulsive thrust. The intermediate pressure compressor compresses the air flow directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive, the high, intermediate and low pressure turbines 16, 17 and 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low pressure turbine 16, 17 and 18 respectively drive the high and intermediate pressure compressors 14 and 13, and the fan 12 by suitable interconnecting shafts.
In view of the above, it will be appreciated that control of the working fluid in terms of airflow through the engine is important for efficiency purposes. In such circumstances, leakage of such working fluid must be reduced and minimised. FIG. 2 is a schematic illustration of a shrouded arrangement of a rotor blade 20. The shroud 23 is upon the tips of the blade 20 and rotates within a cavity 22 formed in a housing 21. Nevertheless, it will be appreciated there is still a leakage path caused by the gap between the end of the shroud 23 and the opposed surfaces of the cavity 22.
Previously, in order to minimise leakage, so called labyrinth seals have been used. The labyrinth seals comprise a series of sharp edges or fins which act to constrict fluid flow leakage through the gap of the shroud 23 and the cavity 22. Generally, there are two types of labyrinth seal, one of a “straight through” type where a succession of upstanding edged fins are provided which extend across the leakage gap in order to minimise it. Alternatively, there is a “stepped” type labyrinth seal in which there are again a succession of upstanding edged fins but the opposed surface is stepped to further provide convolution in flow path and therefore constriction of leakage through leakage gaps provided between opposed parts of the upstanding edged fins and the opposed surface to that fin. Labyrinth seals are well-known and there are a large number of varying geometries and types of which the fins may be inclined and otherwise presented for efficient sealing effect. Nevertheless, generally the edged fins are formed from solid metal with sharp machined edges to maximise the constriction of flow through the leakage gap. It will be understood that this leakage is due to pressure differential across the rotary component which is normally a compressor or turbine in an engine. This pressure differential drives the blades or vanes of the turbine or compressor so that any leakage about the edges of these blades or vanes through the leakage gaps reduces efficiency as this pressurised working fluid thereby leaked provides no work and may present detrimental mixing losses.
The effectiveness of a labyrinth seal is subject to a number of factors. These factors include manufacturing constraints, in service conditions and geometrical limitations. Normally, the clearance between the upstanding fin and its opposed surface is a significant factor with regard to specification of an appropriate seal. This clearance dimension should be as small as possible within the housing but without rotating part clashes or touching during operation. However, the effective area of the seal as depicted in FIG. 3 will ideally be less due to contraction of the leakage flow after fin 30 if that flow separates from a sharp edged thin surface 31. Thus, as can be seen in FIG. 3, the physical clearance between the fin surface 31 and an opposed surface 32 is depicted by gap S whilst the effective gap E is smaller than gap S due to separation of the fluid flow 33 from the surface 31 as it passes between that surface 31 and the opposed surface 32. The reduction in effective seal area (E) is the result of separation from the sharp edge. It is called a “vena contracta”. Compounding the number of this reduced from liquid velocity across the fin because the premium drop is reduced. Multiple constrictions in series reduce the leakage mass flow by reducing the pressure drop across each constriction, hence reducing the leakage velocity through the clearance.